2016 day 4

AOC.Test/Test2016.cs

@@ -9,6 +9,7 @@ public class Test2016
     [DataRow(typeof(Day01), "300", "159")]
     [DataRow(typeof(Day02), "76792", "A7AC3")]
     [DataRow(typeof(Day03), "993", "1849")]
+    [DataRow(typeof(Day04), "361724", "482")]
     public void CheckAllDays(Type dayType, string part1, string part2)
     {
         Common.CheckDay(dayType, part1, part2);

AOC2016/Day04.cs

@@ -3,13 +3,45 @@ namespace AOC2016;
 /// <summary>
 /// Day 4: <a href="https://adventofcode.com/2016/day/4"/>
 /// </summary>
-public sealed class Day04() : Day(2016, 4, "Puzzle Name")
+public sealed class Day04() : Day(2016, 4, "Security Through Obscurity")
 {
-    public override void ProcessInput()
+    private List<Room> _rooms = null!;
+
+    private record Room(string Name, int SectorId, string Checksum)
     {
+        public static Room FromRawLine(string raw)
+        {
+            var s = raw.Split('[');
+            var s2 = s[0].Split('-');
+            return new(string.Join("", s2[..^1]).Replace("-", string.Empty), int.Parse(s2.Last()), s[1].TrimEnd(']'));
+        }
+
+        public bool IsRealRoom() =>
+            Name.GroupBy(c => c)
+                .OrderByDescending(c => c.Count())
+                .ThenBy(c => c.Key)
+                .Take(5)
+                .Select(c => c.Key)
+                .ToArray()
+                .SequenceEqual(Checksum.ToCharArray());
+
+        public string DecryptedName()
+        {
+            var answer = Name.ToCharArray();
+            for (var i = 0; i < Name.Length; i++)
+            for (var l = 0; l < SectorId % 26; l++)
+                answer[i] = answer[i] == 'z' ? 'a' : (char)(answer[i] + 1);
+
+            return new(answer);
+        }
     }
 
-    public override object Part1() => "";
+    public override void ProcessInput()
+    {
+        _rooms = Input.Select(Room.FromRawLine).ToList();
+    }
 
-    public override object Part2() => "";
-}
+    public override object Part1() => _rooms.Where(r => r.IsRealRoom()).Sum(r => r.SectorId);
+
+    public override object Part2() => _rooms.Single(r => r.DecryptedName().Contains("northpole")).SectorId;
+}